A reference source can generate a reference voltage (VREF) and/or a reference current which is independent of power supply and techniques and have an assured temperature characteristic regards of the temperature varies. It would be a criteria to provide a reference source with a low temperature coefficient (TC), low power dissipation and high power supply rejection ratio (PSRR) in the design of integrate circuits, such as an analog-digital converter (ADC), a digital-analog converter (DAC), a dynamic random access memory (DRAM) and a flash memory.
Referring to FIG. 1, a bandgap reference power supply circuit which may carry a curvature compensation for the temperature characteristic. In the power supply circuit, a branch current flowing through a triode Q01 and a branch current flowing through a triode Q02 are positive proportional to absolute temperature (PTAT) currents, while a branch current flowing through resistors R01 and R02 and a branch current flowing through resistors R03 and R04 are negative PTAT currents. Due to the positive and negative PTAT current compensation, the reference voltage VREF has a well temperature drift characteristic. However, the reference voltage may not be very precise because a plurality of resistors is incorporated in the circuit. In case there is a variation in the fabrication process, particularly when the range of the machining angle is exceeded, the resistance of the resistors varies in a rather wide range, such that a significant deviation of the slope for the positive PTAT current to the negative PTAT current occurs, which further leads to the increasing of the PTAT and a lower precised VREF, thus cutting down the performance of the bandgap reference power supply circuit.
One solution for the problems described above is to carry out a second-order curvature compensation for the temperature characteristic to improve the precision of a VREF. Referring to FIG. 2, a bandgap reference power supply circuit with second-order curvature compensation adopting PTAT voltage compensation method is shown. The power supply circuit includes two bandgap reference voltage sources. A first bandgap reference voltage source includes triodes Q1, Q2, Q3 and Q4 and resistors R1, R2 and R3, for generating a PTAT current IPTAT. A second bandgap reference voltage source includes triodes Q5, Q6, Q7 and Q8 and resistors R4, R5 and R6, for generating a reference voltage Vref of the first-order temperature compensation.
Referring to FIG. 2, when a voltage between a base and an emitter of a triode Q10 is lower than its on-state voltage, the two bandgap reference voltage sources are disconnected, and the output reference voltage Vref refers to
      Vref    =                  V                  BE          ⁢                                          ⁢          6                    +                                                  V              T                        ⁢            ln            ⁢                                                  ⁢                          n              2                                            R            4                          ⁢                  (                                    R              4                        +                          2              ⁢                              R                5                                      +                          2              ⁢                              R                6                                              )                      ,in which VBE6 indicates the voltage between a base and an emitter of the triode Q6.
When the voltage between the base and the emitter of the triode Q10 is higher than its on-state voltage, both of the two bandgap reference voltage sources are communicated and the current IPTAT flowing through the triode Q10 can be referred as IPTAT=VT ln n1/R1, therefore, the output reference voltage Vref is
  Vref  =            V              BE        ⁢                                  ⁢        6              +                                        V            T                    ⁢          ln          ⁢                                          ⁢                      n            2                                    R          4                    ⁢              (                              R            4                    +                      2            ⁢                          R              5                                +                      2            ⁢                          R              6                                      )              +                                        V            T                    ⁢          ln          ⁢                                          ⁢                      n            1                                    R          1                    ⁢                        R          6                .            
The triode Q10 is conducted at a predetermined temperature T0, and is cut off when the temperature is lower than T0. The currents flowing through the resistors R3 and R6 both are the PTAT currents, which increased as the temperature rises. When the temperature is lower than T0, a voltage VBE10 between the base and the emitter of the triode Q10 is:
            V              BE        ⁢                                  ⁢        10              =                  2        ⁢                                            V              T                        ⁢            ln            ⁢                                                  ⁢                          n              1                                            R            1                          ⁢                  R          3                    -              2        ⁢                                            V              T                        ⁢            ln            ⁢                                                  ⁢                          n              2                                            R            4                          ⁢                  R          6                      ,in which n1=SQ2/SQ1, n2=SQ6/SQ5, VT is a threshold voltage, and SQ1, SQ2, SQ5 and SQ6 indicate the cross section areas of the triodes Q1, Q2, Q5 and Q6, respectively. Accordingly, when n1=n2 and (R3/R1−R6/R4)>0, VBE10 is increased as the temperature rises. When the temperature equals to T0, VBE10 is equal to the on-state voltage of Q10.
However, the bandgap reference power supply circuit in FIG. 2 has the following problems: (1) there is also a plurality of resistors adopted, when a deviation is caused by variation of the fabrication process, the the resistance of the resistors would widely differs, such that the error coming from the circuit itself may exceed the precision of the second-order curvature compensation, resulting in the failure of the second-order curvature compensation; (2) when the resistance of the resistor R16 wide varies, the triode Q10 may fail to be conducted or there would be an offset for its on-state temperature point; (3) the PSRR performance of the circuit in high frequency section may become worse and therefore could not be incorporated in a high frequency analog circuits (for example, a high-speed ADC circuit); (4) the topological structure of the circuit is relatively complicated.